In general, polyimide resins have high heat resistance and good electrical insulation, and find use in printed circuit boards and heat resistant adhesive tapes. They are also used as resin varnish to form surface protective films and interlayer insulating films for electric parts and semiconductor materials. Polyimide resins, however, are soluble in only a limited number of solvents. Accordingly, it is a common practice to coat a substrate with a solution of polyamic acid which is a precursor for polyimide and is relatively soluble in various solvents, and then to treat the coating at an elevated temperature to effect dehydration and cyclization into a polyimide resin.
Meanwhile, for the purpose of improving the solubility of polyimide resins in solvents, enhancing the adhesion thereof to substrates, and imparting flexibility thereto, an attempt has been made to introduce siloxane chain into polyimide skeleton as disclosed in Japanese Patent Application Laid-Open No.7-268098. However even in this case, the polyimide resin has to be generally diluted with solvent for use. This necessitates a step of removing the solvent by, for instance heating, after the polyimide resin solution is applied or coated to a substrate. Further, measures for keeping good working environment, such as ventilation, are also required.
Japanese Patent Laid-Open Application No.2002-332305 discloses a solvent-free polyimide silicone resin composition. The composition contains a (meth)acrylic compound as a reactive diluent compound to thereby attain high adhesion to a substrate with various shapes and enable one to eliminate a step of removing a solvent after an application process such as coating.
The aforesaid polyimide silicone has drawback that the resin is colored, so that it cannot be applied to an object the color of which is important. To solve this problem, Japanese Patent Application Laid-Open No. 2004-149777 discloses a transparent and colorless polyimide silicone resin obtained mainly form aliphatic tetracarboxylic acid. However, the resin has little compatibility with a reactive diluent and a cured film from a mixture of the resin and the diluent is cloudy.